This invention relates generally to the field of high purity silicon production and within that field to the conversion of metallurgical grade silicon to electronic grade silicon and yet more specifically to a set of three processes for removing impurities from a silicon production facility. One or more of the processes may be used to prevent buildup of impurities in recycle streams and to provide a purer and more stable feedstock to the final purification steps of the standard distillation techniques commonly used to purify a high purity silicon containing gas, which is then used to produce high quality silicon in a deposition reactor, The locations where the impurities are removed are called sinks and the locations where they enter are called sources. Other definitions and abbreviations are;    MCS; monochlorosilane, SiH3Cl    DCS; dichlorosilane, SiH2Cl2     TCS; Trichlorosilane, SiHCl3     STC; Silicon Tetrachloride, SiCl4     MBS; monobromosilane, SiH3Br    DBS; dibromosilane, SiH2Br2     TBS; Tribromosilane, SiHBr3     SBC; Silicon Tetrabromide, SiBr4     MGS; Metallurgical Grade Silicon    EGS; Electronic Grade Silicon    MTCS; Methyl trichlorosilane, Si(CH3)Cl3     MTBS; Methyl tribromosilane, Si(CH3)Br3     H2; Hydrogen gas    HCl: Hydrogen chloride gas    HBr; Hydrogen Bromide gas    Cl2; Chlorine gas    Br2; Bromine gas    Ppm wt is parts per million by weight    Ppma is parts per million atomic    Ppba is parts per billion atomic
The majority of high purity electronic grade silicon is produced from metallurgical grade silicon, MGS, which is approximately 98% pure. The process converts the solid silicon into a liquid material, which can be purified then decomposed back to silicon. The initial material is usually trichlorosilane, TCS, although tribromosilane, TBS, has been used and triiodosilane, SiHI3, could be used. The process involves using carrier chemicals to transform the solid silicon and recycling the carrier chemicals to reduce waste as is shown in FIG. 1. Some carrier chemicals must be used to reject the impurities, which are Carbon, C; Boron, B; Phosphorus, P; Aluminum, Al; and other metals. Minimizing the loss of the carrier chemicals for the waste is also desirable. Carrier chemicals consist of silicon, hydrogen, and a halogen, usually chlorine, in various combinations such as H2, STC,TCS,DCS,MCS, HCl and Cl2, SiH4, H2, etc and in a bromine based system the equivalent bromine analogs. The initial reactors to produce the trichlorosilane were called Siemens reactors and reacted HCl with silicon to form TCS in high, 90%+yield with some STC byproduct, the silicon deposition reactors reacted the TCS with hydrogen to form silicon, some byproduct STC and HCl which was recycled to the Siemens reactor. A detailed example of this silicon refinery approach can be seen in Padovani U.S. Pat. No. 4,213,937, which shows the complexity of the process needed to close the recycle and deal with the byproduct STC. The disposal of the byproduct STC was generally recognized as a problem and various methods arose to convert it back to TCS or to silica and HCl. Ingle, U.S. Pat. No. 4,526,769 shows a process which recycles STC, hydrogen and HCl to a two stage reactor. Breneman, U.S. Pat. No. 4,676,967, shows a process which recycled only STC and hydrogen to a reactor full of MGS and reacted them to form a mixture of TCS, STC, hydrogen and hydrogen chloride and then provided for the progressive disproportionation of TCS to silane, SIH4, and STC which was then recycled to the reactor, the silane was decomposed to silicon and hydrogen, which was recycled. Further information on impurity removal in this same process is provided in Coleman, U.S. Pat. No. 4,340574, which mentions providing a small, 0.01-0.1%, purge stream from the columns used in the disproportionation part of the process.
The prior technology purification approach is primarily distillation of the silicon depositing gas, typically TCS but also TBS and SIH4 to very high purity levels. There are also processes to remove some impurities by using adsorbents. Ingle provides for removal of MGS that is carried over from the reactor and for distillation and chemical purification of TCS. He also provides for distillation of the recycled Silicon tetrachloride. He does not provide for the solid aluminum trichloride and hence his proposed distillation scheme would fail. See column 7 line 2 “The chlorosilanes are separated by distillation in distillation column 78 which separates the lighter boiling constituents (H2SiCl2 and HSiCl3, from the SICl4.” It is apparent from the examples that the research conducted was on the dual stage reactor itself using once through chemicals, Thus the purity and operational problems of a closed recycle system using SICl4 would not have been apparent, and hence were not dealt with in a feasible manner. Ingle also distills all of the recycled SiCl4 which is not required or optimal. Breneman provides for condensing a small portion of the STC from the reactor to trap carried over MGS and metal halides which are then sent to waste and do not require additional dilution prior to hydrolysis and specifically mentions in column 15 line 54 that “Any boron trichloride, boiling point, −12° C., that was not removed in said sludge or retained on the ion exchange resin could be removed from the silane in said purification zone”. Klein et al, U.S. Pat. No. 6,843,972 provides for adsorption of impurities in TCS using a solid base. In the prior technology the primary goal of purification is to remove impurities from the silicon depositing gas. Further goals are that the carrier chemicals are preferably recycled with minimum waste and that the impurities are also rejected with minimum waste of carrier material. Breneman states in column 6 line 44 “All of the byproduct materials are recycled for further use, . . .” The purity levels required are very high, often parts per billion or higher which are very difficult to detect directly and consequently may require use of redundant purification steps. Breneman notes in Col 12 line 60 ” . . . the product silane is of semiconductor purity, having impurities present at parts per billion levels, rather than at levels on the order of about 0.05% or 500 parts per million. It will also be appreciated that such purification steps as indicated herein might in proactive, constitute redundant features useful primarily on that basis”. Coleman claims in column 20 line 30 “the improvement which comprises bleeding a portion of the trichlorosilane-rich bottom stream of (iii) said bleed portion containing one or more of BCl3, PCl3 and AsCl3 impurities and adding said portion to the unreacted silicon tetrachloride bottom recycle stream of (iv) and bleeding a portion of the chlorosilane-rich bottom stream of (viii) said bleed portion containing one or more of B2H6, PH3 and AsH3 impurities and adding said portion to the unreacted silicon tetrachloride bottom recycle stream of (iv), wherein the respective bleed portions of the trichlorosilane-rich bottom stream of (iii) and chlorosilane-rich bottom stream of (Viii) are 0.01 to 0.1 percent of their respective bottom streams.”
A major problem in the prior art is that the purification is primarily of the material typically SiHCl3 or SiH4 or SiHBr3, used for the final silicon deposition. Not surprisingly some of the impurities found in the TCS, TBS or SiH4 are very close chemically and in physical properties to the pure material and the purification required is very high which requires significant loss of product to remove the impurities. Thus distillation columns to remove these impurities tend to be large and expensive to run. Additional columns may be needed because of unexpected difficulties or as redundant systems to compensate for the difficulty of reliably obtaining the needed high purity as noted by Breneman above.
A further problem is that there is no effort to deliberately remove impurities as solids from the effluent gas stream from the reactor. There are some references to removing solid waste, primarily MGS, from the reactor effluent, which consists of a mixture of DCS,TCS STC, hydrogen and hydrogen chloride. Ingle provides a gas solid separator, after the 2nd stage of the reactor, which operates at 300-350° C. and 25-60 psi, whereas Breneman provides for a scrubber where condensation of a small amount of the STC in the stream knocks out the solids. Breneman claims that this stream may contain BCl3, boron trichloride, BP −12° C. but the method of obtaining the purge stream is to condense a small portion of the STC while avoiding condensing the desired TCS. The boiling points of both TCS, 31.7° C. and STC, 57.3° C. are considerably higher than BCl3 so the BCl3 would preferably stay with the uncondensed TCS. These solid removal steps are primarily done to remove solid silicon carryover from the reactor, not to remove impurities
A yet further problem is the problem of impurity buildup in recycle streams. The only notes with regard to the effect of recycle streams on purification is by Coleman where he provides for very small purges, 0.01-0.1%, which are then recycled back to the hydrogenation reactor. As noted earlier Ingle does provide for distillation of the recycle STC but not in a realistic manner since he does not provide for filtration of aluminum chloride despite the fact it is the single largest impurity in the STC stream. Thus here are no realistic processes to remove impurities from the recycle streams other than by purge stream. There are processes to separate the recycle gas streams into hydrogen and hydrogen chloride but both streams are still recycled without any attempt to remove impurities. Since the impurities are not removed they will tend to build up in the recycle loops and thus be in greater concentration in the reactor where they will in turn produce greater concentrations of those impurity species which tend to have similar properties to TCS and thus have to be removed in the expensive TCS distillation system. Hence it can be seen that over a period of time the purity of the TCS will tend to decrease unless the columns are operated at higher reflux ratio but this reduces the throughput and measurement of trace impurities in high purity TCS is very difficult and time consuming so it is hard to control the column at a fixed purity if the inlet impurity concentration is increasing.
A basic deficiency in the prior art is that no system wide analysis is conducted to identify the best locations for the impurity sinks. Frequently the location of the sinks may not be known. The process is very much based on trial and error with copying of techniques, which seem to work without detailed understanding of why they work. Often there can be surprise peaks in impurities after several months of stable operation. The chlorine-based system has a particular problem with the aluminum impurity aluminum trichloride since it does not have a liquid phase and so cannot be distilled. This material tends to form solids, which drop out in tanks, which thus have to be periodically cleaned. This solid is also a Lewis acid and so can form temporary temperature dependent complexes with phosphorus impurities such as PH4Cl which itself is only stable as a solid, decomposing to PH3 and HCl on heating. A period of cold weather may provide high purity then warming may stimulate phosphorus contamination. Alternatively a filter with this material in may be hit with a warmer than usual stream and contaminate it. Similar problems apply to boron contamination. In some processes such as the silane process there is no designated sink for boron, it may go through its own disproportionation process and emerge as diborane, B2H6, to be removed with hydrogenated carbon compounds such as methane, ethane and ethylene from the silane by cryogenic distillation or it may bind to the ion exchange resin catalyst. Carbon is present in TCS as the methyl analogues of chiorosilanes, methylchlorosilane, CH3SiCl3, MTCS dimethylchlorosilane, (CH3)2SiCl2 and the bromine analogs in TBS. It may be removed in distillation, but its chemical similarity to the TCS and STC suggest the likelihood of azeotrope formation and even more difficult separation. In the silane system carbon tracks the silicon in forming methane and other hydrocarbons in the silane from the carbon containing species in the TCS.
The ultimate goal of purification is to remove impurities from metallurgical grade silicon (MGS) and make high purity silicon but the removal of the impurities is not seen as the goal, instead the goal is to make high purity material. Thus a primary deficiency in the prior technology is that the sources and sinks are not closely identified so they may be monitored to prevent surprise buildups. A further major deficiency is that the carrier chemicals are preferably recycled with minimum waste but no effort is made to evaluate impurity buildup or remove impurities from these streams. A yet further deficiency is the failure to evaluate the whole system and select the optimum location for the sinks. A yet further deficiency is the failure to provide for bypasses on recycle stream so that only a portion of the stream need be purified.